Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.
In thermography three approaches are known:
1. Imagewise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element containing other of the ingredients necessary for said chemical or physical process followed by uniform heating to bring about said changes in colour or optical density.
2. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an imagewise heated donor element onto a receptor element.
3. Direct thermal formation of a visible image pattern by imagewise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.
According to U.S. Pat. No. 3,080,254 a typical heat-sensitive (thermographic) copy paper includes in the heat-sensitive layer a thermoplastic binder, e.g ethyl cellulose, a water-insoluble silver salt, e.g. silver stearate and an appropriate organic reducing agent, of which 4-methoxy-1-hydroxy-dihydronaphthalene is a representative. Localized heating of the sheet in the thermographic reproduction process, or for test purposes of momentary contact with a metal test bar heated to a suitable conversion temperature in the range of about 90-150.degree. C., causes a visible change to occur in the heat-sensitive layer. The initially white or lightly coloured layer is darkened to a brownish appearance at the heated area. In order to obtain a more neutral colour tone a heterocyclic organic toning agent such as phthalazinone is added to the composition of the heat-sensitive layer. Thermo-sensitive copying paper is used in "front-printing" or "back-printing" using infra-red radiation absorbed and transformed into heat in contacting infra-red light absorbing image areas of an original as illustrated in FIGS. 1 and 2 of U.S. Pat. No. 3,074,809.
Thermographic materials of type 3 can be rendered photothermographic by incorporating a photosensitive agent which after exposure to UV, visible or IR light is capable of catalyzing or participating in a thermographic process bringing about changes in colour or optical density.
Examples of photothermographic materials are the so called "Dry Silver" photographic materials of the 3M Company, which are reviewed by D. A. Morgan in "Handbook of Imaging Science", edited by A. R. Diamond, page 43, published by Marcel Dekker in 1991.
U.S. Pat. No. 3,152,904 discloses an image reproduction sheet which comprises a radiation-sensitive heavy metal salt which can be reduced to free metal by a radiation wave length between an X-ray wave length and a five microns wave length and being distributed substantially uniformly laterally over said sheet, and as the image forming component an oxidation-reduction reaction combination which is substantially latent under ambient conditions and which can be initiated into reaction by said free metal to produce a visible change in colour comprising an organic silver salt containing carbon atoms and different from said heavy metal salt as an oxidizing agent and in addition an organic reducing agent containing carbon atoms, said radiation-sensitive heavy metal salt being present in an amount between about 50 and about 1000 parts per million of said oxidation-reduction reaction combination.
WO 94/11198 discloses a recording material comprising on a support (i) a heat sensitive layer comprising a substantially light insensitive organic silver salt, (ii) a protective layer containing a matting agent dispersed in a binder and (iii) a reducing agent being present in the heat sensitive layer and/or another layer on the same side of the support carrying the heat sensitive layer. However, the presence of the large quantities of matting agent required to obtain optimum slip properties will in the case of inorganic matting agents lead to premature failure of the thermal head due to abrasion and in the case of organic matting agents lead to image faults due to accumulation of particles on the thermal head. Furthermore, protective layer thicknesses of at least 2 .mu.m are necessary to avoid deformation of the material during printing and particles of matting agent sink into the protective layer during the thermal development process thereby reducing their slip properties. This can be avoided by using larger matting agent particles, but with adverse effects on the thermal contact of the thermal head with the material and on the image quality obtained.
U.S. Pat. No. 4,468,603 discloses a thermographic element comprising a support having coated thereon: (a) a thermographic emulsion layer comprising a non-photosensitive reducible source of silver, a reducing agent for silver ion, and a binder; (b) a layer adjacent to said thermographic emulsion layer comprising a binder and a polymeric fluorinated surfactant; and (c) an outermost layer which is not removed during development of said thermographic element and which is positioned on the side of said support opposite from said thermographic emulsion layer, said outermost layer consisting essentially of a plurality of optically transparent organic polymeric beads. According to the detailed description of U.S. Pat. No. 5,468,603, "The smoothness of the bead surface and shape of the bead are chosen such that the amount of reflected visible wavelengths (400 nm to 700 nm) of light is kept to a minimum. The shape of the beads is preferably spherical, oblong, ovoid, or elliptical. The particle diameter is preferably in a size range of 1-12 .mu.m in average size, more preferably, 1.5 to 10 um in average size; and most preferably 2-9 .mu.m in average size, particularly with fewer than 25% of the total number of beads being outside a range of .+-.15% of the average size of the beads. The beads may be present on the surface from about 50 to 500 beads per square millimeter; more preferably 75 to 400 beads per square millimeter; and most preferably 100 to 300 beads per square millimeter. The increase in percent haze due to the introduction of the beads into the construction is preferably no more than 15%; more preferably no more than 8%; and most preferably no more than 6%. The optically transparent organic polymeric beads which alter the separation or slip characteristics of the element's surface are provided in the imaging layers in such a manner that they tend to protrude from the surface of the outermost layer. The thickness of the outermost backside layers are typically 0.5 to 6 .mu.m." In the invention examples of U.S. Pat. No. 5,468,603, backside compositions are disclosed consisting of 0.5 to 5.8% of polymeric beads, the beads consisting of 7 .mu.m polystyrene methacrylate and 13 .mu.m polymethyl methacrylate beads, 83.5 to 92.7% of cellulose acetate butyrate, 1.2 to 1.3% of a polyester resin, 0.9 to 1.0% of an antihalation dye and 0.08 to 13.4% of antistat L. However, thermal development of thermographic materials with a thermal head is usually carried out with the thermal head in contact with the coating on the thermographic emulsion side of the support. The fluorine-containing surfactant in the outermost layer on the thermographic emulsion side of the support in contact with the thermal head can, at the high temperatures necessary for thermal development, decompose to a small extent resulting in the production of small quantities of hydrogen fluoride, which will attack the outermost layer of the thermal head causing premature failure of the thermal head. Furthermore in the case of photothermographic materials rapid pulsed heating with a thermal head can be used together with image density detection to increase the image density to a predetermined standard level. The presence of a fluorine-containing surfactant in the protective layer of photothermographic recording materials can, therefore, also lead to premature failure of thermal heating components.